Titanium aluminides suffer from the inability to form a self-protective oxidation-resistant barrier on exposure to an oxidizing environment. This is because the alloy tends to form mixed oxide scales that tend to crack upon thermal cycling and form a complex layered oxide that spalls. In addition, the oxide film that is in contact with the metal substrate dissolves some of the oxygen from the oxide scale. This leads to diffusion of oxygen from the surface into the metal substrate and subsequent embrittlement.
In the case of a metal matrix composite (MMC) consisting of high strength filaments embedded in the metal matrix, additional complications arise due to internal stresses generated by the mismatch in thermal expansion between fiber and matrix. This leads to crack formation at the surface during cyclic oxidation, crack propagation into the metal matrix and eventual mechanical failure of the composite.